Mitigation scenarios are usually defined as a description and a quantified
projection of how GHG emissions can be reduced with respect to some baseline
scenario. They contain new emission profiles as well as costs associated with
the emission reduction. Stabilization scenarios are mitigation scenarios that
aim at a pre-specified GHG reduction target. Usually the target is the concentration
of CO2 or the CO2-equivalent concentration of a basket
of gases by 2100 or at some later date when atmospheric stabilization is actually
reached.

There are two common difficulties associated with the formulation and quantification
of mitigation scenarios. First, in certain cases there is not a clear-cut distinction
between intervention and non-intervention scenarios, that is, scenarios with
or without explicit climate policy. This is discussed in detail in Box
2.1. The second important problem regarding mitigation scenarios has to
do with the difference between top-down and bottom-up models. Whereas the latter
focus on engineering trends and technology costs, the former view resource development
from a macroeconomic price-mediated perspective. Although, as discussed in the
SAR (IPCC, 1995), the differences between these approaches are continiously
narrowing as each incorporates elements of the other, there is still quite a
difference in their formulation of emission reduction strategies. This suggests
the importance of including multiple methodological approaches in scenario analysis.

Recent discussions among IPCC experts and reactions from reviewers of
this report and the SRES report revealed the need to clarify differences
between various types of GHG emission scenarios, in particular, between
climate policy scenarios (CP scenarios) and scenarios without climate
policies (NCP scenarios) but with low emissions.

CP scenarios are often, but not always, constructed with reference to
a corresponding reference or baseline scenario that is similar to the
CP scenario in every respect except the inclusion of climate mitigation
measures and/or policies. In fact, climate policy analysis often starts
with the construction of such a reference scenario, to which is added
climate policy to create the CP scenario.

Another type of CP scenario is not originally built around such no-policy
baselines. Developers of such scenarios envision future worlds
that are internally consistent with desirable climate targets (e.g., a
global temperature increase of no more than 1°C by 2100), and then
work backwards to develop feasible emission trajectories and
emission driver combinations leading to these targets. Such scenarios,
also referred to as safe landing or tolerable windows
scenarios, imply the necessary development and implementation of climate
policies, intended to achieve these targets in the most efficient way.

The general definition of CP scenarios provided here enables one to effectively
discriminate between CP scenarios and other scenarios with low emissions
(e.g., IS92c, SRES-B1). Unlike the former, NCP scenarios have low emissions
but do not assume any explicit emission abatement measures or policies,
nor are they designed specifically to achieve certain climate targets.
NCP scenarios by themselves may explore a wide variety of alternative
development paths, including green or dematerialization
futures.

Confusion can arise when the inclusion of non-climate-related
policies in a NCP scenario has the effect of significantly reducing GHG
emissions. For example, energy efficiency or land use policies that reduce
GHG emissions may be adopted for reasons that are not related to climate
policies and may therefore be included in a NCP scenario. Such a NCP scenario
may have GHG emissions that are lower than some CP scenarios.

The root cause of this potential confusion is that, in practice, many
policies can both reduce GHG emissions and achieve other goals. Whether
such policies are assumed to be adopted for climate or non-climate policy
related reasons in any given scenario is determined by the scenario developer
based on the underlying scenario narrative. While this is a problem in
terms of making a clear distinction between CP and NCP scenarios, it is
at the same time an opportunity. Because many decisions are not made for
reasons of climate change alone, measures implemented for reasons other
than climate change can have a large impact on GHG emissions, opening
up many new possibilities for mitigation. Chapters 7, 8, and 9 discuss
ancillary benefits of climate mitigation and the co-benefits of policies
integrating climate mitigation objectives with other goals.

2.2.3 Scenarios and Development, Equity, and Sustainability (DES)

The climate issue is embedded in the larger question of how combined social,
economic, and environmental subsystems interact and shape one another over many
decades. There are multiple links. Economic development depends on maintenance
of ecosystem resilience; poverty can be both a result and a cause of environmental
degradation; material-intensive lifestyles conflict with environmental and equity
values; and extreme socio-economic inequality within societies and between nations
undermines the social cohesion required for effective policy responses.

It is clear that climate policy, and the impacts of climate change, will have
significant implications for sustainable development at both the global and
sub-global levels. In addition, policy and behavioural responses to sustainable
development issues may affect both our ability to develop and successfully implement
climate policies, and our ability to respond effectively to climate change.
In this way, climate policy response will affect the ability of countries to
achieve sustainable development goals, while the pursuit of those goals will
in turn affect the opportunities for, and success of, climate policy responses.

In this report and its Working Group II companion report, climate change impacts,
mitigation, and adaptation strategies are discussed in the broader context of
DES (see Munasinghe, 1999).

The issues raised by a consideration of DES are of particular relevance to
the scenarios discussed in this chapter. Because they are necessarily based
upon assumptions about the socio-economic conditions that give rise to emissions
profiles, mitigation and stabilization scenarios implicitly or explicitly contain
information about DES. In principle, each stabilization or mitigation scenario
describes a particular future world, with particular economic, social, and environmental
characteristics. Given the strong interactions between development, environment,
and equity as aspects of a unified socio-ecological system and the interplay
between climate policies and DES policies, emissions scenarios are viewed in
this report as an aspect of broad sustainable development scenarios.

The allocation of emissions in a scenario is coupled closely to an important
policy question in climate negotiations: the fair distribution of future emission
rights among nations, or burden sharing. For example, an egalitarian
formulation of the rights of developing countries to future climate space
is often expressed in terms of equal per capita emissions allocations. Alternative
assumptions on burden sharing have important implications for equity, sustainable
development, and the economics of emissions abatement. However, it is noteworthy
that this critical conditioning variable is usually not explicitly treated in
mitigation scenarios in the literature (see section 2.3).
Indeed, documentation of scenarios generally does not address the implications
of the scenarios for equity and burden sharing. In rare cases, mitigation scenarios
have been developed which explicitly impose the simultaneous co-constraints
of climate and equity goals (e.g., Raskin et al., 1998).

In this and other ways scenario analysis could become an important way of linking
DES issues to climate policy considerations. However, as discussed in more detail
in section 2.4, many quantitative mitigation and stabilization
scenarios have not been designed with this purpose in mind. As a result, it
is not always easy to draw out the DES implications of particular stabilization
and mitigation scenarios.

Although this chapter focuses on mitigation and stabilization scenarios, it
is important to note that DES issues are also implicit in the base case or reference
scenarios that underlie mitigation and stabilization scenarios. Since the difference
between reference case scenarios and stabilization and mitigation scenarios
is simply the addition of deliberate climate policy, it can be the case that
the DES differences among different reference case scenarios are greater than
between any one such scenario and its stabilization or mitigation version. This
is of particular relevance in the discussion below in section
2.5.2 of scenarios based on the baselines produced in the IPCCs SRES
(Nakicenovic et al., 2000).